This invention is directed to the discovery that enhanced adhesion and shorter cure times can be achieved in certain siloxane resins which have a low degree of organic substitution.
Just recently, there was disclosed in U.S. Pat. No. 3,986,997, issued Oct. 19, 1976 to Harold A. Clark and assigned to the Dow Corning Corporation a new type of abrasion resistant coating composition which consists of a stable dispersion of colloidal silica and a silicone resin. These coating compositions are unique because they are clear coatings, have generally higher abrasion resistance than most organic materials used for the same purpose and are substantially easier to prepare and handle. These coatings are also easier to apply and for most coating purposes they adhere very well to the substrate they are applied to.
There are other silicone resins which have been utilized to coat various substrates for a variety of purposes and whenever there were desirable properties of these resins that those skilled in the art wished to impart to the substrate, there were always ways to gain the necessary adhesion. For example, silicone resins are used on metal substrates for encapsulation purposes but some metals utilized in electrical systems simply would not adhere to the silicone resins. Some of the common approaches to adhesion in these cases was to prime the metal surface with functional silanes such as ##STR1## or (CH.sub.3 O).sub.3 SiCH.sub.2 CH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2 NH.sub.2, allowing an air dry and then applying the resin or, these silanes were added directly to the resin and then the resin was applied to a cleaned metal substrate. Either way, the above noted silanes seemed to provide good adhesion, presumably due to their compatibility in (or with) the silicone resins having a high degree of organic substitution.
When one scans the prior art surrounding the adhesion of silicone resins to various substrates, one continuous thread winds its way through the publications; there is no universal primer or adhesion additive and therefore cautionary statements are always made in those publications that the substrates and the adhesion promoters have to be matched for the best results.
Thus it is not totally unexpected that the ordinary adhesion promoters discussed above would not function well with the siloxane resins having a low degree of organic substitution.
Moreover, when the siloxane resins are being utilized as abrasion resistant coatings, the organosilanes discussed above are not normally utilized because they tend to destroy the hardness of the coating and therefore its effectiveness as an abrasion resistant coating. One specific example of a surface which does not readily adhere to the siloxane resins having a low degree of substitution is polycarbonate. Polycarbonate tends to have inconsistent surface characteristics as it is formed into solid articles and therefore attempts to adhere siloxane resins to such substrates have met with a great deal of difficulty.
What is needed therefore is a means to obtain consistent and uniform adhesion to troublesome substrates so that abrasion resistant siloxane coatings having a low degree of substitution can be adhered to such substrates.